1. Field of the Invention
The present invention relates to a spread illuminating apparatus, and particularly to a spread illuminating apparatus used as an illuminating means for a liquid crystal display.
2. Description of the Related Art
A liquid crystal display characterized by its low profile, small occupied volume and light weight has been extensively used in electric products including mobile phones and personal computers, and the demand thereof has been increased. However, since a liquid crystal of the liquid crystal display does not emit light by itself, an illuminating means is needed besides the liquid crystal display when used in dark places where solar or room light is not satisfactorily available. Thus, this illuminating means is required to be compact in size and small in power consumption. In recent years, a thin plate-like spread illuminating apparatus of side light type is often applied.
FIG. 4 shows an embodiment of a conventional spread illuminating apparatus of side light type.
As shown in the figure, a spread illuminating apparatus 1xe2x80x2 is generally composed of a transparent substrate 2 made of a light-transmissible material and a bar-like light source 5 disposed close to an end surface 8 of the transparent substrate 2. Light rays emitted from the light source 5 are introduced into the transparent substrate 2 to illuminate a liquid crystal display (not shown) disposed under the transparent substrate 2.
The light source 5 is composed of a bar-like light conductive member 3 made of a transparent material and at least one spot-like light source (for example, a light-emitting diode) 4a (4b) disposed facing one end 6 (7) of the light conductive member 3. The light conductive member 3 is provided with an optical path conversion means 12xe2x80x2, which includes grooves shaped, for example, triangular in section and formed on a surface opposite to a surface 9 facing the end surface 8 of the transparent substrate 2 so as to allow light rays emitted from the spot-like light source 4 to be made incident on the end surface 8 of the transparent substrate 2 in a substantially uniform manner. The light conductive member 3 is disposed with the surface 9 facing the end surface 8 of the transparent substrate 2 at a predetermined distance therefrom.
A light reflection member (frame) 13 is provided around the light conductive member 3 so as to efficiently guide the light rays emitted from the spot-like light source 4a (4b) into the transparent substrate 2. The light reflection member 13 is substantially U-shaped, covers longitudinal surfaces of the light conductive member 3 except the surface 9 facing the transparent substrate 2 and is made of a hard resin having a metal (such as silver) vapor-deposited film, a white film, etc. stuck on a surface (an inner surface) facing the light conductive member 3, or made of a bent aluminum or stainless steel sheet.
A light reflection pattern 49 is formed on an upper surface 46 of the transparent substrate 2. The light reflection pattern 49 comprises grooves 47 shaped triangular in section and formed running parallel to the light conductive member 3 with predetermined intervals from each other and flat portions 48 present adjacent thereto. The grooves 47 are formed with respectively different depths so that light emitted from the light conductive member 3 is reflected in a substantially uniform manner everywhere at the surface of the transparent substrate 2 irrespective of the distance from the light conductive member 3 (the light source 5) thereby uniformly illuminating a liquid crystal display (not shown) provided under the transparent substrate 2. Specifically, the depth of the groove 47 increases gradually in proportion to the increase in distance from the light conductive member 3.
However, in an illuminating means using the light reflection pattern 49, when light is reflected at the light reflection pattern, interference fringes (moire pattern) caused by fringes due to the difference in transmissivity between the grooves 47 and the flat portions 48 and by an array of the mosaic pattern of liquid crystal cells constituting pixels of the liquid crystal display (liquid crystal display element) become conspicuous, thereby making the image difficult to observe. The generation of the moire pattern is related closely to the pattern shape of the light reflection pattern 49, i.e., the depth, size and interval of the grooves 47
The countermeasure against the moire pattern was disclosed in a publication (Monthly Display, April Issue, 2000) describing that the moire pattern is most effectively eliminated by forming the grooves with a predetermined inclination angle xcex8 (xcex8=22.5xc2x0) against the longitudinal direction of the light conductive member. A spread illuminating apparatus based on the above configuration is shown in FIG. 5.
As shown in FIG. 5, a light reflection pattern 59 formed on an upper surface of the transparent substrate 2 comprises grooves 57 and flat portions 58 adjacent thereto, and the grooves 57 are formed inclinedly to have a predetermined inclination angle xcex8 (preferably xcex8=22.5xc2x0) with respect to the optical axis of the bar-like light source 5.
However, a spread illuminating apparatus 1xe2x80x3 in FIG. 5 has the following problems though it comes up with a countermeasure effective against the moire pattern.
The transparent substrate 2 suffers a reflective image of the spot-like light source 4a due to the inclination of the grooves 57 as well as the position of the spot-like light source 4a on an end surface 6 of the light conductive member 3, whereby a strip-shaped bright line 51 is generated crossing the grooves 57 from one end surface 8 of the transparent substrate 2 toward another end surface 18 opposite to the end surface 8. Specifically, a dark area A defined by the bright line 51, a side surface 52, and the end surface 18 appears on the surface of the transparent substrate 2 and makes itself conspicuous especially against the bright line 51, thereby degrading the illumination quality. Here, the reflective image of the spot-like light source 4a in the transparent substrate 2 refers to the phenomenon in which the stripe-like bright line 51 is observed in the transparent substrate 2 when observed from a viewing surface (upper surface) due to the fact that light emitted from the spot-like light source 4 is introduced into the transparent substrate 2 in a stripe via the end surface 6 of the light conductive member 3.
This occurs, because the grooves 57 are formed so as to have the inclination angle xcex8 as shown in FIG. 5, and thus light from the light source 5 is scarcely guided to the grooves 57 located in the area A, and also because the grooves 57 are inclined so as to gradually get closer to the end surface 8 of the transparent substrate 2 from one side surface 52 of the transparent substrate 2 toward another side surface 53 and have their depths gradually increased in proportion to the increase in distance from the end surface 8 while one groove has a constant depth all the way, and thus the light introduced into the transparent substrate 2 is reflected in a smaller amount at the grooves positioned toward the side surface 52 of the transparent substrate 2 than at the grooves positioned toward the side surface 53 thereby providing the area A with low brightness and an area B with high brightness.
The present invention has been made in light of the above problems, and the object of the present invention is to provide a spread illuminating apparatus which can suppress the generation of the moire pattern and bright line and ensure uniform brightness over the whole screen.
In order to solve the above problems, according the present invention, in a spread illuminating apparatus which comprises: a transparent substrate made of a light-transmissible material; and a light source comprising a bar-like light conductive member and spot-like light sources disposed on both ends of the light conductive member and disposed along and close to an end surface of the transparent substrate; in which an optical path conversion means comprising a plurality of grooves and a plurality of flat portions adjacent thereto is provided on a side surface of the light conductive member; the depth of the grooves increases in proportion to the increase in distance from each of the spot-like light sources and keeps increasing up to a predetermined position where a deepest groove is formed; and in which a light reflection pattern comprising a plurality of grooves and a plurality of flat portions adjacent thereto is formed on the transparent substrate with the grooves having a predetermined angle with respect to the optical axis of the light source; the predetermined position of the deepest groove is shifted from the center of the light conductive member toward either of the spot-like light sources.
Light rays emitted from the spot-like light sources disposed on both ends of the light conductive member are introduced into the light conductive member, reflected at inclined surfaces of the grooves and at the flat portions constituting the optical path conversion means, and guided into the transparent substrate. With the deepest groove positioned off the center of the light conductive member as described above, the intensity of the light rays guided into the transparent substrate from the light conductive member is not symmetric with respect to the center of the light conductive member, thereby leveling off the unevenness in brightness (difference in brightness) conventionally having appeared on the transparent substrate.
Generation of moire pattern can be held by structuring the light reflection pattern such that its grooves are formed to have a predetermined inclination angle with respect to the optical axis of the light source, and the unevenness in brightness, which occurs on the transparent substrate due to the above described structure of the light reflection pattern, can be significantly diminished by shifting the position of the deepest groove off the center of the light conductive member.